Our invention relates to a process and apparatus for observing or monitoring pressing forces in a tablet press and, more particularly, for monitoring the pressing forces due to upper and lower pistons of a tablet press acting pairwise.
In the known process for monitoring pressing forces of upper and lower pistons of a tablet press acting pairwise in a piston pair, an average value is formed from successive measurement results of the pressing force and is compared with two adjustable tolerance bounds. The average value so formed is correlated with two adjustable individual value limits which follow the course of the average value or are fixed, with which maxima of the pressing force measurement results are compared.
Tablet presses are used for making tablets, which have upper and lower pistons in pairs, which press the powdered material in matrices for making tablets. Thus operating problems of different types can occur. These problems can include piston breaking. Also they can be due to differing piston lengths or partial slight ruptures on the piston head surfaces or also because of dirt. Metering fluctuations occur in operation which are caused by nonuniformities in the nature of the material being processed.
To detect resulting variations in the final product tablet promptly, tablet presses generally are provided with a pressing force monitoring apparatus. One such apparatus has standard strain measuring gauges, which are mounted on supports of the plattens for the pistons. Thus the conditions allowing different pressing forces result in different strains and stresses on the platten support which are converted into a proportional voltage signal.
For detection of errors an average value A is formed with a known device from successive measurement values, whose course is followed within two adjustable fixed tolerance bounds, G. As soon as the average value A exceeds or does not fall within these tolerance bounds G the tablet press is stopped. All causes for operational problems may not be detected in time or promptly with such a device however since a slight damage to an individual piston of the machine does not substantially change the average value in a machine with a plurality of pistons. Moreover with such a device a piston break already present in a machine in operation can not be established, since such a fault is not subsequently clearly detectable in a clear shift of the average value. Besides in this type of device the tolerance limits must be selected comparatively far apart to prevent the tablet press being frequently stopped without any problems present. That is because among other things the pressing force of the individual piston pairs differs in its size because of the manufacturing tolerances in the length of the piston so that accordingly the individual values from which the average value is formed fluctuate. A comparatively large separation of the tolerances from each other leads however to detection of only comparatively large deviations of the individual values of the pressing force from the average values and particularly to accompanying operation problems.
To provide a known device (German Patent 25 20 662) to correct that, two adjustable individual value limits B and C are correlated with the average value A and follow its course. The maxima of the individual values of the pressing forces are compared to these individual value limits B and C. This has the advantage that fluctuations of the average value within an adjustable tolerance range between the limits G, which are independent of a second tolerance range defined by the limits B and C for the variations of the maxima of the individual values of the pressing forces, can be included. Thus a comparatively narrow tolerance range can be chosen, because the fluctuations of the average value have no influence on the tolerance range given by the limits B and C for the variations of the maxima of the individual values of the pressing forces on the average value A.
Although piston breaks are detectable with this known process without further features or efforts because of the occurring fluctuations in the pressing forces, however minimal changes in the pressing force of an individual piston pair, which are due to ruptures on the piston head surfaces or to buckling or adhering and/or dirt on the piston head surfaces, are not, because piston faults of this kind have less effect on pressing force fluctuations than unavoidable metering fluctuations or platten force variations which mask such piston faults.
On the other hand buckling of the ram, partial rupture on the tool and adhering residues have an undesirable influence on the piston head surfaces on the strength, the decomposition rate and on the shape of the tablets. Advantageously it is therefore desirable to detect faults of this kind in the automatic manufacture of tablets and to be able to halt production automatically.